1. Field of the Invention
This invention relates to a method of manufacturing a liquid crystal display panel, and particularly to a method of manufacturing a liquid crystal display panel in which liquid crystal is injected by a dispenser-injection method.
A liquid crystal display panel is formed by a pair of glass substrates with liquid crystal sealed therebetween. Thin film transistors, pixel electrodes, alignment films and the like are formed on the surfaces facing each other of the glass substrates. The alignment of the liquid crystal is controlled for each pixel electrode to display an image. Since a liquid crystal display panel is thinner, lighter and lower power consumption than that of a CRT display, it is broadly used in electronic equipments such as personal computers. However, as the panel size increases, it becomes difficult to maintain in-plane uniformity of visual quality and a solution to this difficulty is desired.
2. Description of the Related Art
To maintain in-plane uniformity of visual quality in a liquid crystal display panel, liquid crystal must be evenly sealed between two glass substrates. In an ordinary liquid crystal sealing technique, after a frame-like seal material is formed on the peripheral edge of one of the substrates, spacers made of spherical glass particles, glass fibers cut into short pieces or the like are scattered on the substrate and then the two substrates are laminated via the seal material and dipped into liquid crystal in a vacuum atmosphere. As an atmospheric pressure is restored in this state, the liquid crystal is injected from a liquid crystal injection port provided in the seal material because of the difference in pressure. After that, the liquid crystal injection port is sealed.
However, in the above-described technique, the liquid crystal enters in between the substrates only through the liquid crystal injection port provided on the peripheral edge of the panel. Therefore, if the panel size is increased, injection of liquid crystal takes longer and it becomes difficult to secure uniformity.
Thus, a dispenser-injection method for a liquid crystal has been used recently. In this method, after a frame-like seal material is formed on the peripheral edge of one substrate, spacers are scattered on the substrate. Then, after liquid crystal is dropped on the substrate, the other substrate is aligned on the substrate while being pressurized. This evenly spreads the liquid crystal between the substrates. In this state, the substrates are laminated via the seal material and the liquid crystal is sealed therein.
According to this dispenser-injection method, which enables even spreading of the liquid crystal between the substrates of large areas within a relatively short time, many spacers are evenly scattered on the substrate before sealing the liquid crystal as described above in order to maintain an even gap between the substrates. However, when the two substrates are laminated while being pressurized and the liquid crystal is spread between them, the spacers shift together with the liquid crystal and the distribution of the spacers becomes uneven. As a result, a problem arises that an even gap cannot be maintained between the substrates.
To prevent the spacers from shifting on the substrate, using spacers coated with an adhesive has been proposed. In this technique, however, the adhesive coated on the spacers disturbs the alignment of the liquid crystal and may cause display defects.
Thus, a technique of forming resin protrusions on the substrate to maintain the gap between the substrates instead of scattering spacers has been proposed (See JP-A-6-194615).
FIGS. 3A to 3E are sectional views of processes for explaining a method of manufacturing a liquid crystal display panel using this technique. In FIG. 3A, the reference numeral 1 represents one of a pair of glass substrate and the reference numeral 2 represents the other. Thin film transistors, pixel electrodes and wiring patterns for connecting these are formed on the surface of the glass substrate 1, and transparent electrode and color filters are formed on the surface of the glass substrate 2, though not shown for simplification.
First, as shown in FIG. 3A, an acrylic resin film 3 is uniformly coated on the glass substrate 2 and resist patterns 4 are formed by an ordinary photolithography process. Then, the acrylic resin film 3 is etched using the resist patterns 4 as masks. Pillar spacers 5 made of the acrylic resin film 3 are thus formed, as shown in FIG. 3B. In the case where the acrylic resin film is photosensitive, the acrylic resin film itself can be patterned by the photolithography process to form the pillar spacers.
Next, the surfaces of the glass substrates 1 and 2 are cleaned. This cleaning process includes water cleaning and subsequent optical cleaning. The optical cleaning is carried out to improve the cleaning effect of the water cleaning and also to improve the wettability of an alignment film formed by a later process. The optical cleaning is carried out by irradiating light of a low-pressure mercury lamp from above the glass substrates 1 and 2, as shown in FIG. 3B.
Next, alignment films 6 and 7 are formed on the surfaces of the glass substrates 1 and 2, respectively, and heat-treated, as shown in FIG. 3C.
Next, a seal material 8 is formed on the peripheral edge of the glass substrate 1, as shown in FIG. 3D. Then, liquid crystal 9 is dropped on the surface of the glass substrate 1 on which the seal material 8 has been formed. After that, the surfaces of the glass substrates 1 and 2 are caused to face each other and the glass substrates 1 and 2 are laminated in this state, as shown in FIG. 3E.
According to the above-described technique, the spacers can be evenly arranged on the substrate by patterning using the ordinary photolithography technique. Moreover, since the arrangement of the spacers can avoid pixel positions, disturbance of the alignment of the liquid crystal near pixels due to overlapping of the spacer arrangement positions and the pixel positions can be prevented. Furthermore, as the resin film with an even thickness is coated on the substrate, the height of the spacers can be made uniform. The acrylic resin such as polymethacrylate is convenient because it can be easily patterned using the photolithography technique and can be provided with sufficient mechanical strength and chemical resistance as spacers by the heat treatment after the patterning.
The visual quality of the liquid crystal display panel is largely affected by the uniformity of the gap between the substrates. In the above-described technique of forming the pillar spacers made of the resin film, the gap between the substrates is defined by the height of the pillar spacers. This can be controlled by coating the resin film to a uniform thickness as designed.
FIG. 4 is a view for explaining the influence of the height of the pillar spacers on the visual quality of the liquid crystal display panel. The same elements as those in FIGS. 3A to 3E are denoted by the same numerals. As shown in FIG. 4, if the height of the pillar spacers 5 are lower than the height of the seal material 8 formed on the peripheral edge of the substrate, the gap between the substrates is relatively increased in the peripheral part of the panel, generating so-called frame-edge unevenness. On the other hand, if the pillar spacers 5 is taller than the seal material 8, the inner volume of the gap between the substrates becomes larger than a liquid crystal dropping amount and the liquid crystal cannot sufficiently fill the gap between the substrates, generating so-called a bubble trouble. Therefore, the height control of the pillar spacer 5 must be strict. Normally, the thickness of the coated resin, which defines the height of the pillar spacers 5, is set within an allowable range of ±100 nm with respect to a designed value. If the height of the pillar spacers 5 can be set within this allowable range with respect to the designed value, the lowering of the visual quality due to the above-described frame-edge unevenness or the like can be restrained to a practically trouble-free level.
However, the inventor has discovered that even in the case where the resin film is coated to a thickness within the allowable range, the height of the pillar spacers is reduced through a later liquid crystal display panel forming process, generating the above-described frame-edge unevenness. This makes it difficult to achieve a gap of a designed value between the substrates. It is a major cause of lowering the visual quality.